In the design of ICs for processing analog signals, there is often a need to include reconfigurable analog filters, whose filter bandwidths are variable and adjustable according to given applications. Particularly, using reconfigurable analog filters is advantageous for implementing wireless communication receivers due to the following reasons. First, there is a trend in using a single receiver for communications with different wireless communication systems running under different standards (e.g., 3G, WiFi, WiMax), and radio signals under different standards have different signal bandwidths. Reception of these signals by a single receiver requires either multiple analog filters with different bandwidths, or a single analog filter which bandwidth can be tunable to match these different signal bandwidths. The latter approach results in a significant saving of chip area in the design of ICs. Second, there is an increasing use of variable-bandwidth assignment in mobile communication systems for supporting services with different quality requirements and also for optimizing the system capacity. In this assignment scheme, the bandwidth of the signal is variable and changes over time. This condition prompts the use of reconfigurable analog filters in the implementation of radio receivers.
A reconfigurable analog filter can be realized as a Gm-C filter. A tutorial overview on the Gm-C filter and its architectures can be found in E. Sanchez-Sinencio and J. Silva-Martinez, CMOS transconductance amplifiers, architectures and active filters: a tutorial, IEEE Proceedings on Circuits, Devices and Systems, vol. 147, no. 1, pp. 3-12, February 2000; the disclosure of which is incorporated herein by reference by its entirety. A Gm-C filter comprises a plurality of transconductance amplifiers and capacitors. A transconductance amplifier is to convert an input voltage or a voltage difference into a current with magnitude proportional to the input voltage or the voltage difference. The current-to-voltage ratio is known as the transconductance value. Usually, all or most of the transconductance amplifiers of a Gm-C filter share a common transconductance value. The bandwidth of the Gm-C filter can easily be tuned by adjusting this transconductance value. In IC realization, the transconductance value is made variable and is controlled by an external voltage source, referred herein to as a tuning voltage. Adjusting the tuning voltage to a desired value that in turn correctly set the filter bandwidth can be accomplished by the following technique.
A voltage controlled oscillator (VCO) is an oscillator which oscillation frequency is controlled by an external voltage source, herein referred to as control voltage. The VCO can be realized by transconductance amplifier(s) and capacitor(s) organized in certain form of feedback loop, with the transconductance amplifier(s) and the capacitor(s) being identical to the ones used in the Gm-C filter. The oscillation frequency of such VCO is related to the transconductance value of the transconductance amplifier(s) and the capacitance value of the capacitor(s) by a first relationship that can be derived according to the circuit theory.
The bandwidth of the Gm-C filter can then be determined by the same transconductance value and the same capacitance value based on a second relationship. The teaching of how to determine this second relationship is disclosed in Jaime E. Kardontchik, Introduction to the Design of Transconductor-Capacitor Filters, Springer, 1992; the disclosure of which is incorporated herein by reference by its entirety. It follows that by setting a certain oscillation frequency for the VCO, it leads to a corresponding filter bandwidth for the Gm-C filter. Corresponding transconductance value used for setting both the VCO frequency and the filter bandwidth can be obtained. A phase locked loop (PLL) can be used to align the VCO frequency with a reference frequency. The voltage used to control the VCO frequency can also be used as a tuning voltage to set the transconductance value of the transconductance amplifiers in the Gm-C filter. Note that the PLL is a tuning circuit for the Gm-C filter. The operating principles of PLLs and their circuits can be found in Donald R. Stephens, Phase-Locked Loops for Wireless Communications: Digital, Analog and Optical Implementations, Kluwer Academic Publishers, 2002, and in Floyd M. Gardner, Phaselock Techniques, John Wiley and Sons, 1979. The disclosures of both references are incorporated by reference herein by their entireties.
The IC implementation of a Gm-C filter tuning circuit imposes certain requirements. First, the setting of the filter bandwidth should be accurate and independent of the variation of IC component parameters, such as transconductance and capacitance values; the variation of the supply voltage provided to the IC; and the change in the operating temperature. Second, the circuit topology should be simple requiring less IC chip area in turn reducing the cost of mass manufacturing of the ICs. Third, the power consumption of the IC should be kept minimal. Conserving battery power is an important criterion for handheld portable communication devices such as mobile phones.
In U.S. Pat. No. 6,727,768, a VCO used in a tuning circuit for a Gm-C filter is disclosed. This VCO comprises six resistors with four different resistance values: RA, RB, RD and RE. The VCO frequency then depends on RB*(RE+2RA)/(RB+2RD)/RE. The disadvantage of this dependency is that the sensitivity of the VCO frequency to the inaccuracy of these resistance values is large. Moreover, buffers or bypassing capacitors are required to stabilize the voltage references generated by the resistor network, thereby increasing the IC chip area and the power consumption.
U.S. Pat. No. 7,239,197 discloses a tuning circuit. In this circuit, however, a dedicated multi-phase clock generator is required, introducing uncertainty in the filter bandwidth due to phase inaccuracy. Also, certain reference voltages are involved in the charging and the discharge processes of capacitors. Small errors in these reference voltages can affect the oscillation frequency. Buffers or large bypassing capacitors are also required, hence increasing the IC chip area.
U.S. Patent Application No. 2007/0096798 discloses another tuning circuit. The peak detector therein (i.e. the phase frequency detector) comprises two fully differential comparators, which consume a substantial amount of power and IC chip area. Moreover, the tuning voltage generated by the tuning circuit is proportional to an expression dependent on certain reference voltages, and tuning error can occur when these reference voltages drift due to die-to-die variation in component parameters, and variations in supply voltage and temperature.
In Y. H. Kim and H. K. Yu, Automatic tuning circuit for Gm-C filters and published in the 12th IEEE International Conference on Electronics, 2005, a tuning circuit is disclosed. In this tuning circuit, the VCO frequency depends on a supply voltage and a number of resistors. As a result, the tuning voltage is highly sensitive to the variations in the supply voltage and the resistors. The two transconductance amplifiers used also increase the IC chip area.
In U.S. Pat. No. 7,863,945, the oscillation frequency of the VCO depends on the resistance values of a plurality of resistors. It follows that variation in component parameters can have a profound effect on the accuracy of the oscillation frequency.
As can be seen in current state of the art, there is a need for an improved tuning circuits that fulfill the aforementioned three requirements: (1) robustness against variations in IC component parameters, supply voltage, and operating temperature, (2) minimal IC chip area implementation, and (3) low power consumption.